Storm Forecast
Valid: Sun 23 May 2021 09:00 to Mon 24 May 2021 06:00 UTC
Issued: Sun 23 May 2021 09:31
Forecaster: GATZEN

A level 2 was issued across north-eastern Italy, Slovenia, northern Croatia, and western Hungary mainly for severe wind gusts and to a lesser extent tornadoes and large hail.

A level 1 was issued across Hungary and western and southern Ukraine mainly for severe wind gusts, large hail, and tornadoes.

A level 1 was issued across north-eastern Poland into the southern Baltic area mainly for severe wind gusts.

A level 1 was issued across north-western Russia mainly for severe wind gusts and tornadoes.

A level 1 was issued across the northern Caucasus region into southern Russia mainly for large hail and severe wind gusts.

SYNOPSIS

A strong mid-level jet is still present between low geopotential from southern-western to Central Europe and a ridge across the Mediterranean. Rapidly progressing short-wave troughs are present across the Baltic States and over central Europe. Farther south, cut-off troughs are centered across Iberia and Turkey. Another intense Atlantic trough enters north-western Europe.

Low-level moisture is pretty weak over most places, with some enhanced moisture located along the main frontal boundary from the west Mediterranean towards the Ukraine. Rich low-level moisture is only expected to the north of the Caucasus region. However, in the highly dynamic situation, some areas of organized storms are expected, although the development is quite uncertain in the high-shear, low-CAPE environment.

DISCUSSION

North-eastern Italy, Slovenia, northern Croatia, Hungary, northern Romania and southern Ukraine

At the southern flank of a short-wave trough that crosses Germany, the strong mid-level jet and associated tropopause fold crosses the Alps today. Models indicate that 900 to 700 hPa lapse rates will increase across northern Italy before noon, atop of a moist boundary layer air mass. First storms have developed over the Alps already and are expected to spread south in the next hours together with an outflow boundary that is expected to reach the Adriatic Sea in the afternoon. Weak CIN, low LFC, and rather moist profiles support the suggestion that surface-based storms can develop along this boundary, despite extremely strong vertical wind shear (deep layer shear up to 50 m/s) and a rather cool boundary layer due to many clouds currently limiting diurnal heating. These limiting factors indicate a high forecast uncertainty at this time.

A band of convective storms is expected with some isolated supercells forming at its southern edge. Although rapid storm merger is expected, with right moving storms likely moving with the southward spreading boundary, tornadoes are not ruled out that may be strong. The main threat, however, is severe wind gusts with the band of convective storms. Furthermore, locally large hail is forecast.

Storms are forecast to spread east rapidly with the progressing short-wave trough. Profiles look less favorable for severe storms east of Hungary, with less boundary layer moisture and increasing CIN in the evening. However, MUCAPE persists far into the Ukraine and convective storms are forecast well into the night or even morning with the passage of the trough and upper-level tropopause fold. In the strongly-sheared environment, severe wind gusts are forecast to be the main threat.

North-western Russia

Ahead of the filling low pressure system, a southerly flow evolves ahead of a cold front that moves into western Russia. It advects a warm air mass towards north-western Russia. Modest lapse rates will overlap with enhanced low-level moisture of more than 10°C dewpoints within the warm advection regime and models indicate a narrow band of some 100s J/kg MLCAPE during the day along the main warm front. This warm front will likely intensify due to diabatic frontogenesis and also confluence beneath low- to mid-level synoptic-scale lift ahead of the trough and within the warm advection regime.

Convection initiation is expected due to weak CIN and frontal lift. Storms that form south of the warm front are expected to be rather weak due to limited CAPE and low-level moisture. Storms that form within the warm front have a higher potential to organize due to long and curved low-level hodographs. Multicells and supercells are forecast, with storm motion vectors pointing towards the cold air. Strongly right-moving storms will have the best potential to stay within the best overlap of moisture, lapse rates, lift, and shear, posing a threat of large hail, severe wind gusts, and tornadoes. Ongoing large-scale lift will support an active warm front with numerous storm clusters that become gradually elevated during the evening and night that can go on until the morning while severe threat will weaken.

Germany to Poland and Baltic States

In the wake of the low pressure center, a cool and rather dry air mass will experience some 100 J/kg MLCAPE in response to diurnal heating. Weak CAPE will limit the potential of well-developed storms, however, strong low-level vertical wind shear, in particular across Belarus into western Russia can support some shallow multicells and mesocyclones, capable of producing severe wind gusts and tornadoes.

Georgia to southern Russia

Ahead of the quasi-stationary cut-off trough across Turkey, a southerly mid-level flow persists east of the Black Sea. Steep mid-level lapse rates overlap with a rather deep moist boundary layer that becomes increasingly unstable in response to diurnal heating, along he moderate CIN and rather high LFSs. Convection initiation is likely over the Caucasus mountains during the day with storms merging and spreading north along the outflow boundaries. Moderate vertical wind shear of 10 to 15 m/s in the lowest 6 km will allow for multicells and isolated supercells are forecast in areas with enhanced shear. CAPE in the hail growth layer supports a large hail threat with such storms. Additionally, severe wind gusts are possible along bowing segments of the storms.

North-western Europe

An Atlantic trough progresses across northwestern Europe. Despite weak CAPE, some showers and thunderstorms are forecast to develop late in the period that can produce severe wind gusts along bowing lines due to strong low-level vertical wind shear.